Blending of pigments has a long history. 32,000 years ago prehistoric artists used blends of ground red or yellow ochre with clays, charcoal, juice of berries and fat for making paintings on cave walls and ceilings. Currently, blending of various pigments takes place in many industries and in fine arts. Main purpose of blending of pigments is mixing of pigments with different colors to get another, composite color. The blending is brought to a level of high sophistication, with a considerable control of the color properties of the final products.
For industrial applications, as well as in fine arts, permanence and stability of inorganic, organic, or special pigments and their blends are highly desirable. Such attributes as heat stability, toxicity, tinting strength, staining, dispersion, opacity or transparency, resistance to alkaline or acid, interaction between pigments as well as lightfastness (resistance to discoloration caused by light exposure), determine their suitability for particular manufacturing processes and applications. Chemical or electrochemical degradation of pigments typically cause economical losses.
Various methods have been used in the industry to strengthen the corrosion resistance of pigments, including deposition of protective coatings on the top of pigment particles, and/or addition of passivators and corrosion inhibitors to the ink or paint vehicle. For example, Li et al. in US Patent Application Publication 2008/0314284 disclose highly anti-corrosive thin platelet-like metal pigments, in which the surface of thin platelet-like metal substrates are treated with phosphoric acid compounds and/or boric acid compounds, and are further coated with a layer containing hydrated tin oxide to improve the corrosion resistance. Detrimentally, the passivated pigments of Li et al. are more costly than their non-passivated counterparts, due to additional labor and materials costs.
Goniochromatic optical interference pigments, also termed as color-shifting interference pigments, provide bright, vivid colors due to their multilayered interference structure. Optical interference pigments containing a metallic reflector layer and one or more semi-transparent absorber layers are most color-effective among all known high performance pigments. However, these pigments are highly sensitive to the exposure of corrosive media.
Protective coatings can be applied to color-shifting interference pigments. For example, Phillips in US Patent Application Publication 2004/0160672 disclose color-shifting multilayer interference pigments with the outer layers of silicone dioxide that function as a protective layer for the core optical structure C/SiO2/C. In a paint or ink composition that may be subjected to abrasion in a delivery system, the SiO2 outer layers are known to prevent abrasion to the core optical structure that gives rise to color. Thus, in this instance, the color in the paint or ink composition is more durable. Vuarnoz et al. in U.S. Pat. No. 7,381,758 disclose a passivated optically variable pigment, and suitable passivating compounds for this pigment, including anionic tensides.
Corrosion resistance of goniochromatic interference pigments can also be improved by heat treatment of pigment particles. For example, Phillips et al. in U.S. Pat. No. 5,569,535 disclose a collection of color-shifting interference thin film platelets of high chroma. In order to impart additional durability to the interference platelets, the latter can be annealed or heat treated at a temperature ranging from 200° C.-300° C., and preferably from 250° C.-275° C., for a period of time ranging from 10 minutes to 24 hours, and preferably a time of approximately 15-30 minutes.
Phillips et al. in U.S. Pat. No. 5,570,847 and US Patent Application Publication 2002/0160194; and Bradley et al. in U.S. Pat. Nos. 6,157,489; 6,243,204; and 6,246,523 disclose a method of heat-treating multilayer interference platelets to improve durability of the platelets, including subjecting the platelets at a temperature of 200°-300° C. for 10 minutes to 24 hours. The platelets are formed from a multilayer color-shifting interference thin film construction comprising a metal reflecting layer having a multilayer interference thin film structure on both sides of the metal reflecting layer. The multilayer interference thin film structure includes a pair of layers consisting of a dielectric layer and a semi-opaque metal layer with the dielectric layer of the pair being directly adjacent to the metal reflecting layer. However, the pigments of Phillips et al. and Bradley et al. require the extra step of heat treatment at elevated temperatures.